1. Field
Disclosed herein is a two-component structural adhesive composition based on organic compounds containing radically polymerizable multiple bonds, and an adhesive formed therefrom, and method for providing bonding between a rare earth permanent magnet and a substrate.
2. Description of Related Art
The construction of magnet systems from rare earth permanent magnets frequently requires adhering multiple magnets to one another or to support parts, for example magnetic yokes, particular demands being placed on the adhesive system due to the chemical and physical properties of the magnets.
Rare earth permanent magnets, in particular those made of neodymium-iron-boron, are highly susceptible to corrosion under the effect of humidity or condensation. For neodymium-iron-boron materials, moisture results in planar formation of rust on the magnet surface (“red corrosion”). In addition, the neodymium-rich phase is attacked, this attack being particularly dramatic due to the fact that the neodymium-rich phase in the crystal structure performs the function of a mechanical binder between the magnetic particles. As a result of this so-called “white corrosion,” the magnet decomposes into a grayish-white powder. The corrosion of the neodymium-rich phase is associated with the uptake of hydrogen and formation of neodymium hydride, which further accelerates the spread of the white corrosion. Water uptake occurs through reaction with ambient moisture (humidity, condensation), and is relatively slow at room temperature. At elevated temperatures, for example 80° C. and higher, the reaction proceeds at an appreciable rate. In both cases the corrosion is significantly accelerated in the presence of acids. Thus, at room temperature the reaction rate is relatively high for white as well as red corrosion, and within a few weeks may result in destruction of the magnet. Therefore, acid-free adhesive systems are required for designing magnet systems composed of rare earth permanent magnets.
Furthermore, for the manufacture of magnet systems using adhesion, in particular when the magnets are installed in the magnetized state, the use of rapid-set adhesive systems is advantageous since use of complicated holding devices until the adhesive is cured may be avoided, and mass production assembly may be carried out quickly and reliably using pick-and-place machines and automated adhesive machines. For many magnet systems, such as rotors for electric motors and generators, for example, multiple magnets must be placed directly adjacent to one another, and it is essential to achieve reliable curing of the adhesive to a minimum strength in a short period of time in order to prevent slippage or separation of the magnets due to magnetic forces of attraction or repulsion. The aim is to achieve sufficient curing with a minimum adhesion shear strength of 0.5-1.0 N/mm2 in less than two minutes, in order to allow reliable fitting of the system carrier parts with magnetized magnets.
A further important requirement imposed on the adhesive system used is thermal resistance of the adhesion under typical operating conditions of the magnet systems, which for use of motors, for example, means a thermal load of up to 150° C. In addition, the magnet adhesion should absorb stresses which act on the adhesive during curing or for temperature fluctuations, for example as the result of differences in the thermal expansion coefficients of the adhered materials, thereby preventing cracks or impaired adhesion in the adhesive film.
Various adhesive systems are known for adhering materials, in particular rare earth permanent magnets. For example, cyanoacrylate adhesives, also known as “instant adhesives,” are known which thus easily meet the criterion of rapid curing and setting. A disadvantage of cyanoacrylate adhesives is that they must be processed in a climatized adhesion room having a defined humidity. A further disadvantage of these adhesives is that the adhesions do not have long-term resistance under load. Furthermore, the adhesions are sensitive to moisture and the adhesive itself is relatively brittle, as the result of which the adhesive strength decreases markedly after temperature fluctuations. Such adhesives may generally be used only up to an operating temperature of 80° C.
Heat-curing epoxy resin systems, which have high strength and resistance, are widely used for adhering workpieces. However, these adhesives must be cured using continuous ovens having residence times of up to one hour, or, for more rapid curing, by the use of complex and costly induction techniques. Such epoxy resin systems are not particularly suitable for adhering electronic components such as rare earth permanent magnets, for example.
The use of two-component structural adhesives based on substituted acrylates or methacrylates has been known for quite some time and is very prevalent. Such adhesives cure very rapidly, and have high mechanical and thermal load capacity. However, the disadvantage of these adhesives is that rapid curing requires more or less high concentrations of acrylic, methacrylic, or other organic acids, such as carboxylic acids, which are used to improve adhesion to the substrates by salt formation, which is associated with release of metal ions which act as accelerators for the curing. Typical acid contents of the two-component structural adhesives are 1 to 10% by weight. This relatively high acid content makes the adhesives unsuitable for direct adhesion of neodymium-iron-boron magnets.
Thus, there remains a need for a magnet adhesive for rare earth permanent magnets which combines lack of acid, thermal resistance, rapid curing, and high strength.